In 1951, Klingsberg1 et al described the use of P4S10 dissolved in pyridine as a thionating agent. Pyridine and P4S10 react readily to form a zwitter-ionic, non-smelling compound, the composition of which, P2S5.2C5H5N, was studied as early as 1967-1968 by German inorganic chemists2,3 who obtained evidence for its structure by 31P NMR data4 as well as by comparison with related molecules.
In spite of the teachings of Klingsberg et al., the predominantly used agent in the reaction of thionation of compounds containing an oxo group has been the so-called Lawesson's reagent (IUPAC name: 2,4-bis(4-methoxyphenyl)-1,3,2,4-dithiadiphosphetane-2,4-dithione), herein below referred to as LR. LR was introduced in 1968 for transformations in organic chemistry and was used with a considerable number of reactants, such as amides and ketones, which were thionated in fair yields. However, LR as a thionating agent suffers from a number of drawbacks. For example, its thermal stability is mediocre; it has even been reported that LR starts to decompose above 110° C.5,6. Further, LR has a generally low solubility, which quite often has necessitated the use of hexamethylphosphoramide (HMPA) as a solvent. HMPA is suspected of being carcinogenic to humans and its use is prohibited in many countries. Additional drawbacks with LR are the strong, unpleasant smell of the compound in itself and the fact that during a reaction, there tends to be formation of foul-smelling side-products that are difficult to separate from the desired reaction products (column chromatography is often required).
It appears that there still remains a need for an improved process for the thionation of an oxo group-containing compound as well as an improved thionating agent for use in such process.